The present invention relates to a method for dewatering the web in a paper or board machine. The invention also relates to an apparatus for carrying out the method.
Within the end zone of a wire section upstream of a pick-up suction roll, paper and board machines are generally provided with a wire suction roll in an effort to give the web a sufficiently high dry matter content upstream of the press section. This also creates a sufficiently strong web prior to the action occurring in the press section.
One such solution is disclosed in U.S. Pat. No. 4,075,056, wherein the wire suction roll guides the wire and a web lying on top of it within the end zone of the wire section as they are curving upon said suction roll in a certain sector towards a pick-up suction roll. In line with the suction sector provided by the wire suction roll there is a press roll, a so-called lump-breaker roll, placed thereagainst from the side of a web in an effort to seal the web against the wire suction roll, to consolidate the forming web, and to reduce the amount of fiber lumps in the web.
On the other hand, U.S. Pat. No. 3,846,233 discloses a twin-wire paper-making machine wherein, within the end zone of a twin-wire dewatering zone, water is removed by means of a suction roll at which the lower wire deflects towards a pick-up suction roll while water is simultaneously removed through the upper wire as a result of tension of the wires as well as centrifugal force.
In modern paper and board machines, however, the wire suction roll is one of the major consumers of energy because it requires a vacuum system.
It is also expensive in terms of its construction. The wire suction roll also creates a noise problem during operation of the machine. In addition, the use of a vacuum reduces the web temperature, which is harmful in the press section. All the above problems associated with suction are further emphasized due to the fact that the increasing machine speeds require an increased vacuum capacity.